(19)
J
(12)
Europat sl^^SPdtentamt
European Patent Office
Office europ^en des brevets (11) EP 0 794 599 A2
EUROPEAN PATENT APPLICATION
(43) Date of publication:
10.09.1997 Bulletin 1997/37
(21) Application number: 97103864.1
(22) Date of filing: 07.03.1 997
(51) intCI-^: H01S3/06. H04B 10/12
CM
<
in
o
Q.
(84) Designated Contracting States:
DE FR GB IT
(30) Priority: 07.03.1996 J P 50654/96
(71) Applicant: FUJITSU LIMITED
Kawasaki-Shi, Kanagawa 211 (JP)
(72) Inventors;
• Stiimojoh, Naomasa
Nakahara-ku, Kawasaki-shi, Kanagawa 211 (JP)
(54) Gain equalizer which Includes a plurality
amplifier
(57) A gain equalizer which equalizes gain versus
wavelength characteristics of an optical amplifier. The
gain versus wavelength characteristics of the optical
amplHier include first, second and third gain peaks in a
wavelength band with the second gain peak being
between the first and third gain peaks. The optical
amplifier amplifies an input signal in accordance- with
the gain versus wavelength characteristics to produce
an output signal. The gain equalizer includes first, sec-
ond and third optical filters having first, second and third
transparency characteristics, respectively. The first,
second and third transparency characteristics are peri-
odic waveforms having different periods related to the
wavelength difference between the first and third gain
peaks. The second transparency characteristic is a peri-
. Terahara, Takafumi
Nakahara-ku. Kawasakl-shi, Kanagawa 211 (JP)
• Chikama, Terumi
Nakahara-ku, Kawasakl-shi, Kanagawa 211 (JP)
(74) Representative:
Ritter und Edier von Rschern, Bernhard, DipL-
Ing. et ai
Hoffmann Eitle,
Patent- und Rechtsanwaite,
Arabellastrasse 4
81925 Munchen (DE)
of optical filters for equalizing the gain of an optical
odic waveform having a period equal to 1/(2") of the
period of the waveform of the first transparency charac-
teristic. The third transparency characteristic is a peri-
odic waveform having a period equal to 1/4 of the period
of the waveform of the first transparency characteristic.
For example, the first transparency characteristic is a
periodic waveform with 1/4 period extending between
the first and third gain peaks, the second transparency
characteristic is a periodic waveform with one period
extending between the first and third gain peaks, and
the third transparency characteristic is a periodic wave-
form with two periods extending between the first and
third gain peaks.
I INPUT
10
FIG. 3
'"gain equalizer"'
-20
FILTER *
FILTER
- FILTER
r-Hr12 ^20-1 ^20-2 ^20-;
'21
iOUTPUT
PD
PUMPING-LO
CONTROLLER
•14
^16
Printed by Rank Xerox (UK) Business Services
2 14.UC4
IS
EP0 794 599 A2
Description
rnnaft-RPFFRFNCE TO RPI atfh APPLICATIONS
5 This application is based on. and claims priority to, Japanese application number 08-050654, filed on March 7.
1 996, in Japan, and which is incorporated herein by reference.
RA( 7KnRnLJND OF THE INVENTION
10 1 . Field of the Invention
The present invention relates to a gain equalizer for equalizing the gain of an optical amplifier. More particularly, the
present invention relates to a gain equalizer which includes a plurality of optical filters with transparency characteristics
represented by periodic waveforms having different periods.
2. Description of the Related Art
FIG 1 is a diagram illustrating a conventional optical communication system which uses wavelength division mul-
tiplexing (WDM) to increase the transmission capacity of the system. Referring now to FIG. 1 . a plurality of optical send-
?o ing stations (OS) 1 000#1 to 1000#n produce individual signals having different wavelengths of XI to Xn, respectively
where n is an integer. The individual signals are provided to a multiplexer (MUX) 1002 which multiplexes the individual
signals together into a single WDM signal provided to a transmission line 1 004. Transmission line 1004 is often a single
The WDM signal propagates through transmission line 1004 and is received by a demultiplexer (DEMUX) 1006.
25 Demultiplexer 1006 demultiplexes the WDM signal light back into individual signals having different wavelengths of a1
to xn. respectively Each individual signal is then provided to a corresponding optical receiving station (OR) 1008#1 to
When transmission line 1004 is relatively long (that is. the WDM signal is to be transmitted over a "o"9 Jslance) a
Plurality of repeaters 1012 must be Inserted into transmission line 1004 to amplify the WDM signal as the WDM signal
30 travels through transmission line 1004. A repeater is typically referred to as a "submarine" repeater when rt is for use
underwater in a ti-ansmission line extending, for example, between continents.
Each repeater 1012 typically includes an optical amplifier 1014. which is often an erbium-doped fiber amplifier
(EDFA) to amplify the WDM signal. Generally, an EDFA uses an erbium-doped fiber (EDF) as an amplifying medium
AS the WDM signal travels through the EDF, pump light is provided to the EDF from a pump light source (not illustrated)
35 so that the pump light interacts with, and thereby amplifies, the WDM signal. m k , hoc^ notarial
An EDFA has gain versus wavelength characteristics based on the composition of an optical fiber base mate lal
used to make the EDR These gain versus wavelength characteristics are not perfectly flat in a wavelength band of 1 .5
to 1 6nm the band generally used for long distance optical transmission. Therefore, an EDFA typically experiences an
undesirable "gain tilt", where the individual signals in ttie WDM signal are amplified with different gains, depending on
40 the power of the pump light For example, when the power of the pump light is relatively high, the EDFA may produce a
negative gain tilt, where higher wavelength components in the WDM signal are amplrtied less than lower wavelength
components in the WDM signal. Similarly, when ttie power of the pump light is relatively low, the EDFA may produce a
positive gain tilt, where higher wavelength components in the WDM signal are amplified more than lower wavelengtti
components in the WDM signal. Thus, tiie gain tilt of an EDFA may not be flat.
45 When a transmission line extends for a relatively long distance (such as, for example, between continents) it is usu-
ally necessary to insert tens of stages of submarine repeaters in series into the transmission line. Therefore the WDM
signal will be amplified by tens of stages of optical amplWiers in series. Unfortijnately, when a WDM signal is transmitted
through tens of stages of optical amplifiers, the cumulative effect of gain tilt in the optical amplifiers will cause dispersion
of optical signal-to-noise ratios (SNRs) of the individual signals in the WDM signal. Such dispersion will result in a low
50 Optical SNR Which will be further degraded in each subsequent repeater.
For example FIG. 2(A) is a graph illustrating an optical spectrum waveform where a WDM signal is conventionally
transmitted through ten (10) repeaters in series, and FIG. 2(B) is a graph Illustrating an optical ^Pectrum waveform
where a WDM signal is conventionally transmitted ttnrough sixty (60) repeaters in series. In both cases, an Al-low-den-
sity (less than 1 Wt%) EDF is used, and tiie WDM signal includes four individual signals at diHerent wavelengths multi-
plexed togettier^ FIG. 2(A), in the case of ten (10) repeaters in series, the dispersion of optical SNR is relatively smalL
However, as shown in FIG. 2(B), in the case of sixty (60) repeaters in series, the dispersion of optical SNR is increased
and thereby results in individual signals having an insufficient optical SNR.
Various methods have been proposed to compensate for the dispersion of optical SNR when a large number of
2
BNSOOCIO; <EP P7»4S99A2J_>
tEP 0 794 599 A2
lission line. For example, one such proposed method is to use an Al-low-density
(less than 1 Wt%) EDF and a Fabry-Perot etalon optical filter as a gain equalizer. See Takeda. et al.. "Gain equalization
of Er-doped fiber amplHier using etalon filter", a publication of autumn communication society by the Institute of Elec-
tronics. Information and Communication Engineers. 1995. B-759.
5 A second proposed method allows the transmission of twenty (20) waves on a 6300-km path using an Al-iow-den-
sity EDF and a fiber grating filter as a gain equalizer. See, for example. N. S. Bergano et al.. "lOO/Gb/s WDM Transmis-
sion of Twenty 5Gb/s NRZ Data Channels Over Transoceanic Distances Using a Gam Flattened Amplifier Cham , Th.
A. 3. 1. ECOC'95. . , .... -
A third proposed method is to use a Mach-Zehnder type gain equalizer with an Al-low-density optical amplifier. See
w Kazuhiro Oda et al.. "16-Channel x 10-Gbit/s Optical FDM Transmission Over a 1000 km Conventional Single-Mode
Fiber Employing Dispersion-Compensating Fiber and Gain Equalization", OFC 95. PD22-1 to PD-22-5.
In each of the above-described proposed methods, generally, a gain equalizer has reverse transmission character-
istics in relation to the amplifier characteristics of an EDFA. Thus, the gain equalizer compensates for the amplifier char-
acteristics to produce a flatter amplifier gain in a narrow band, thereby obtaining a flat transmission band and reducing
fs dispersion of optical SNR. The WDM signal is transmitted in the narrow band.
Generally, the narrow band is set as a 10 nm band of 1550 to 1560 nm. The narrow band is limited in size since,
generally, the above-described proposed methods can only compensate for the gain of an EDFA around a single gam
peak in the gain characteristics of the EDFA, or along a small portion of positive or negative gain slopes of the gam char-
acteristics of the EDFA. Therefore, the narrow band for transmission allowed by the above-described proposed meth-
20 ods is too narrow to transmit a WDM signal which includes a relatively large number of individual signal lights.
gi IMI^ARV OF THF INVENTION
Accordingly, it is an object of the present invention to provide a gain equalizer which will sufficiently flatten gain ver-
2S sus wavelength characteristics of an EDFA over a relatively large band when tens of repeaters are connected in series.
Preferably, such a relatively large band can include, for example, several 10-nm bands of 1.53-|im band to 1.56-Mm
''^"Additional objects and advantages of the invention will be set forth In part in the description which follows, and, in
part will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing an apparatus, such as a gam equalizer,
for equalizing gain versus wavelength characteristics of an optical amplifier. The gain versus wavelength characteristics
have first and second gain peaks in a wavelength band with a wavelength difference between the first and second gam
peaks The apparatus includes first and second optical filters connected to the optical amplifier and having first and sec-
ond transparency characteristics, respectively The first and second transparency characteristics are periodic wave-
forms having periods related to the wavelength difference between the first and second gain peaks. The period of the
waveform of the first transparency characteristic is different from the period of the waveform of the second transparency
characteristic. i. u • i ^
Objects of the present invention are also achieved by providing an optical communication system which includes
an optical amplifier and first and second optical filters. The optical amplifier has gain versus wavelength characteristics
40 With first and second gain peaks in a wavelength band and a wavelength difference between the first and second gain
peaks The first and second optical filters have first and second transparency characteristics, respectively. The first and
second transparency characteristics are periodic waveforms having periods related to the wavelength difference
between the first and second gain peaks. The period of the waveform of the first transparency characteristic is different
from the period of the waveform of the second transparency characteristic. The first and second optical filters each filter
45 either an input signal to the optical amplifier or an output signal of the optical amplifier.
Objects of the present invention are also achieved by providing a method for equalizing gam versus wavelength
characteristics of an optical amplHier which amplKies an input signal in accordance with the gam versus wavelength
characteristics, to produce an output signal. The gain versus wavelength characteristics have first and second gain
peaks in a wavelength band with a wavelength difference between the first and second gam peaks. The method
50 includes the steps of (a) filtering either the input signal or the output signal with a first transparency characteristic, and
(b) filtering either the input signal or the output signal with a second transparency characteristic. The first and second
transparency characteristics are periodic waveforms having periods related to the wavelength difference between the
first and second gain peaks. The period of the waveform of the first transparency characteristic is different from the
period of the waveform of the second transparency characteristic.
55 Objects of the present invention are further achieved by providing an additional method for equalizing gam versus
wavelength characteristics of an optical amplifier. The gain versus wavelength characteristics have first and second
gain peaks in a wavelength band with a wavelength difference between the first and second gain peaks. The method
includes the steps of (a) branching a light signal into first and second signals, (b) filtering the first signal with a first trans-
parency characteristic, (c) filtering the second signal with a second transparency characteristic, and (d) combining the
30
35
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:<EP _0794S99A2J_>
EP 0 794 599 A2
70
30
35
50
55
11
filtered first and second signals intSfSombined signal which is amplrtied by the optical amplifier The ti'sta"^ second
t Insp^^cy chLart^^^^^^ are periodic waveforms having periods related to the wavelength difference be^^een the
Jrand se^^^^^^^^ peaks. Moreover, the period of the waveform of the first transparency charactenstic .s different
from the period of the waveform of the second transparency characteristic.
in addition objects of the present invention are achieved by providing a method which includes the steps of (a
branchi^rouSg^ of Tn optical amplifier into first and second signals, (b) filtering the first signa wrth a first
Jansoarencv chaTacterLic (c) filtering the second signal with a second transparency characteristic, and (d) combining
theSSfand^^^^^^^
ac^IScIa JpericSc w^^^^^^ having periods related to the wavelength difference between first and second gain
Ss d the g^n versus wavelength characteristics of the optical ampl^ier. Moreover, the period of the waveform of he
first tTOre'ry characteristic is different from the period of the waveform of the second transparency charactenstic.
RRIFF DESCRIPTinN OF TH P DRAWINGS
Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are
illustrated in the accompanying drawings, wherein like reference numerals refer to like ^ ...
RC3 1 (prior art) is a diagram illustrating a conventional optical communication system which uses wavelength divi-
sion multiplexing (WDM) to increase the transmission capacity of the system. »..^„^„l„oH
pTa 2(A) is a graph illustrating an optical spectrum waveform where a WDM signal is conventionally transmitted
*n^a 2(B;2 L'^ Jph HlusS'ng an optical spectrum waveform where a WDM signal is conventionally transmitted
through sixty (60) repeaters in series.
FIG 3 is a d agram illustrating a gain equalizer according to an embodiment of the present invention
fIg 4 1; a diagram illustrating a Fabry Perot etalon filter, according to an embodiment of the present invention.
""raTr:ph"gtco^^^
third optical filters Slustrated in FIGS. 5(B), 5(C) and 5(D), respectively, according to an embodiment of the present
'"'^ FIG aP) is a graph illustrating a resulting waveform of the amplifier characteristics illustrated in FIG. 5(A) and the
combined waveform illustrated in FIG. 5(E), according to an embodiment of t^e Present .nvention.
FIGS 6(A). 6(B) and 6(C) are graphs illustrating amplHier characteristics of an EDFA for small, medium and large
aluminum (Al) densities, respectively, according to an embodiment of the present invention.
Tra y is a graph illustrating amplHier characteristics of an Al-low-density (less than 1 Wt%) EDFA, according to an
'"^^rsiia^ss:^^^^^^^^^
"TGlTstgZ'E:;^^^^^^^
"•irG^ot'SirsS^^^
a case where Fabry^Perot etalon filters of 32-nm and 16-nm periods of wavelength are connected in series, according
to an embodiment of the present invention. . u «
fSTL a graph illu'strating transparency characteristics of a gain equalizer (GEO) and an equaling resul . a
case where Fabry-Perot etalon filters of 56-nm, 28-nm and 14-nm periods of wavelength are connected in series,
according to an embodiment of the present invention. ai u „i, .j-:>nci*w prfa
FIG. 12 is a diagram illustrating a gain equalizer for equalizing amplifier characteristics of an Al-high-density EDFA,
according to an embodiment of the present invention.
FIG. 13 is a diagram illustrating a gain equalizer for equalizing amplifier characteristics of an Al-high-dens.ty EDFA.
according to an additional embodiment of the present invention. ^,„,„^^Kr-^i
FIG. 14 is a diagram illustrating a modification of the gain equalizer illustrated in FIG. 13. according to an embodi-
"TriS^a d^^r^^^^^^^^ a modification of the gain equalizer illustrated in FIG. 13. according to an embodi-
ment of the present invention. < ^ ai u:«h n^ncitu frfa
FIG. 16 is a diagram illustrating a gain equalizer for equalizing amplifier characteristics of an A!-high-density tUhA.
4
BNSOOCID: <EP p794599A2J_>
EP 0 794 599 A2
according to a further embodiment of tlie present invention.
FIG. 17 is a diagram illustrating an optical transmission system which includes a gain equalizer, according to an
'"^sr^sTadt^^^^^^^
ing to an additional embodiment of the present invention.
ncQ r.RiPTinN Of THE P BFPFRRF=D EMRODIMENTS
Reference will now be made in detail to the present preferred embodiments °V'^PTtiT.lT,^Zhou?
which are Illustrated In the accompanying drawings, wherein like reference numerals refer to like elemente throughout
Fia 3 is a diagram illustrating a gain equalizer according to an embodiment of the f>'^^^'^ZT"-^T!!2nZ
to FIG. 3. a WDM signal is received at an input 2 of an optical transmission line 5 and Is provided to an Al-h^h-densrty
,5 EDFA 10 Pump ligW produced by a pumping laser diode (LD) 12 is also provided to Al-»^f 'd^"^' ^J^^^Vo Jhm
pump light InteracS an EDF forming Al-high<lensity EDFA 10 to cause the WDM signal to be amplrf.ed as the WDM
K fravels through Al-high-density EDFA 10. More specifically, the WDM signal travelling through Al.h,gh<Jens,ty
EDFA 10 is amplified by energy provided from the pump light produced by laser diode 12. u „u^„ncih,
The amplified WDM signal then travels through a gain equalizer 20 which equalizes the gam of Al-h,gh-density
20 EDFA 10 The eauallzed. amplified WDM signal is then provided to an output 21 of transmission line 5.
A Jortln oHhSifled'wDM signal light is decoupled from transmission line 2 and provided to a photo defector
(PD) 14 to monitor characteristics of the amplified WDM signal. Based on the value of the
pumping-LD controller 16 controls output energy of laser diode 12 to fix the output power of the ^"^Pl'^'f^^^^, JfnaL
' Sain equalizer 20 includes a plurality of optical filters 20-1 , 20-2. and 20-3 «)nnected in series, to «,uahze the
ampl«ier characteristics of Al-high.lensity EDFA 1 0 over a wide wavelength band. Each opt^^^^^^ 20a ^^^■^'^J^'
3 is preferably a Fabry-Perot etalon filter, although the present invention is not intended to be limited to the use of Fabry |
piS ?alS fiSe^ and other types of filters may be appropriate. More specifically, any type of filter which provides
appropriate filtering characteristics may be used as optical filters 20-1 , 20-2 and 20-3.
Arthough FIG. 3 illustrates gain equalizer 20 as being positioned at the output side of Al-h.gh-density EDFA 1 0. gam
30 eaualizer 20 can be positioned at the input Side of Al-high-density EDFA 10.
equalizer 20 ca p ^^^^ ^ ^^^^^^ ^^^^^ ^^^^^.^ ^, T^^Z
Referr^g now to FIG. 4. a Fabry-Perot etalon filler has a spacer layer 100 sandwiched between high-reflection multi-
?^e!Z, Z ln6 104. High-reflection multilayer films 102 and 104 form a resonator in which a resonance condition
is determined by an angle of incidence e of an incident light 106 and a thickness L of spacer layer 100.
35 When a wavelength of incident light 1 06 satisfies the resonance condition of the resonator, ^'--^J^^^^^^^
light 106 is maximized. Therefore, the filter will receive incident light 106 and will pass various el^"9^^°"^P°;!"^
in passed light 108 (which is a combination of multiple lights) and will reflect -^'-'^^^ wavelength am^^^^^^^
ref tected ligW 1 1 0 (which is a combination of multiple lights). In this filter, the angle of incidence e and the thickness L
of spacer layer 100 are predetermined. The transparency characteristics of the filter may P«"<f '^^''^.J^^^ Pf^^^/
40 predetermined wavelength interval. Therefore, by forming the filter while adjusting the angle o the incidence 6 and he
thickness L of spacer layer 100, the transparency characteristics of the filter may be periodically varied over a predeter-
""Sllisr^XstratingamplHier Characteristics Of
amplrtier characterises includes three peaks, peak 1, peak 2 and peak 3. The amplitude '^J °' fj^^ .P^^^^J '^^^
45 peak 3 are generally different, and the wavelength interval between adjacent peaks is substantially equal. Thus, the
wlveiength Len^al between peak 1 and peak 3 is substantially equal to the wavelength interval between peak 3 and
pelkl T will be discussed in more detail further below the amplifier characteristics illustrated in FIG. 5(A) , ncU.de
Siree peai (as opposed to. for example, two peaks) since a high density of Ai is used m the EDFforming the EDFA.
A^ Med in FIG. 5(A). the ampWier characteristics of Al-high density EDFA 10 has periodical characteristics m
50 a rel^am band Therefore by use of a Fourier transform, the amplifier characteristics may be substantially transformed
to Sanctions. Accordingly, as discussed in more detail below, periodic transparency characteristics of o^^^^^^^^^
filters 20-1, 20-2 and 20-3 can be combined to form reverse characteristics of the amplifier characteristics of Al-high-
"'"tTorfs^ecLly. FIG. 5(B) is a graph illustrating transparency characteristics of optical ™f ^0-^;^^^^^^^^^^^^
55 embodiment of the present invention. As illustrated in FIG. 5(B), the transparency characteristics of optK:al fHter 20
are represented by a periodic waveform having a period related to a wavelength difference between peaks m the amph-
fi^ SaSScs d'^-high-density EDFA 10. In the present example, the period of the waveform of the transparen y
characteristics is determined in accordance with the wavelength difference between peak 1 and peak 2^or example^
as illustrated in FIG. 5(B). the transparency characteristics of optical filter 20-1 are represented by a periodic waveform
5
EP 0 794 599 A2
with 1/4 period extending between 1 and peak 2.
FIG. 5(C) is a graph illustrating transparency characteristics of optical filter 20-2, according to an embodiment of
the present invention. As illustrated in FIG. 5(C), the transparency characteristics of optical filter 20-2 are represented
by a periodic waveform having a period which is one-half the period of the transparency characteristics of optical filter
5 20-1 . For example, as illustrated in FIG. 5(C), the transparency characteristics of optical fitter 20-2 are represented by
a periodic waveform with one period extending between peak 1 and peak 2.
FIG. 5(D) is a graph illustrating transparency characteristics of optical filter 20-3. according to an embodiment of
the present invention. As illustrated in FIG. 5(D). the transparency characteristics of optical filter 20-3 are represented
by a periodic waveform having a period which is 1/4 the period of the waveform of the transparency characteristics of
10 optical filter 20-1 . For example, as illustrated in FIG. 5(D), the transparency characteristics of optical filter 20-3 are rep-
resented by a periodic waveform with two periods extending between peak 1 and peak 2.
FIG. 5(E) Is a graph illustrating a combined waveform of the transparency characteristics of optical filters 20-1, 20-
2 and 20-3 illustrated in FIGS. 5(B), 5(C) and 5(D), respectively according to an embodiment of the present invention.
More specifically as shown in FIG. 5(E), by connecting optical filters 20-1, 20-2 and 20-3 together in series (as illus-
15 trated In FIG. 3), a combined waveform is formed having substantial reverse characteristics of the amplifier character-
istics of Al-high-density EDFA 10.
FIG. 5(F) is a graph illustrating a resulting waveform of the amplifier characteristics illustrated in FIG. 5(A) of Al-
high-density EDFA 10 and the combined waveform illustrated in FIG. 5(E). according to an embodiment of the present
invention. In this manner, the amplifier characteristics of Al-high-density EDFA 10 having three gain peaks may be
20 equalized in a wide wavelength band.
In the transparency characteristics illustrated in FIGS. 5(B), 5(C). 5(D) and 5(E), a peak-to-peak amplitude of a
periodical waveform corresponds to an attenuation degree of the corresponding filter.
Therefore, according to the above embodiments of the present invention, a gain equalizer equalizes gain versus
wavelength characteristics of an optical amplifier. The gain versus wavelength characteristics of the optical amplifier
25 include first, second and third gain peaks in a wavelength band with the second gain peak being between the first and
third gain peaks and a wavelength difference between the first and third gain peaks. The optical amplifier amplifies an
input signal in accordance with the gain versus wavelength characteristics to produce an output signal. The gain equal-
izer includes first, second and third optical filters having first, second and third transparency characteristics, respec-
tively. The first, second and third transparency characteristics are periodic waveforms having different periods related
30 to the wavelength difference between the first and third gain peaks. For example, the second transparency characteris-
tic is a waveform having a period equal to 1/(2") of the period of the waveform of the first transparency characteristic.
The third transparency characteristic is a waveform having a period equal to 1/4 of the period of the waveform of the
first transparency characteristic.
More specifically for example, as illustrated in FIGS. 5(A), 5(B). 5(C) and 5(D), the first transparency characteristic
35 is a periodic waveform with 1/4 period extending between peak 1 and peak 2. The second transparency characteristic
is a periodic waveform with one period extending between the peak 1 and peak 2, The third transparency characteristic
is a periodic waveform with two periods extending between peak 1 and peak 2.
FIGS. 6(A), 6(B) and 6(C) are graphs illustrating amplifier characteristics of an EDFA in relation to the aluminum
(Al) densities in the EDF More specifically FIG. 6(A) is a graph illustrating amplifier characteristics of an EDFA for small
40 aluminum (Al) densities of less than 1 Wt%. FIG. 6(C) is a graph illustrating amplifier characteristics of an EDFA for high
aluminum (Al) densities of greater than approximately 4 Wt%. FIG. 6(B) is a graph illustrating amplifier characteristics
of an EDFA for medium aluminum (Al) densities between the 1 Wt% and 4 Wt%.
From FIGS. 6(A). 6(B) and 6(C), it can be seen that, as the Al density increases from a low-density condition of less
than 1 Wt%. a peak 2 forms in the 1 555-nm gain band and extends toward a short wavelength side. When the Al density
45 increases to a high-density condition of more than approximately 4 Wt7o. the 1 .545-^m gain band rises, and a peak 3
forms.
FIG. 7 is a graph illustrating amplifier characteristics of an Al-low-density (less than 1 Wt%) EDFA, and FIG. 8 is a
graph illustrating amplifier characteristics of an Al-high-density (more than 1 Wt%) EDFA.
In the gain versus wavelength characteristics of the Al-high-density EDFA shown in FIG. 8, a minimum value of the
50 gain in the 1 .54-|nm band is smaller and the gain versus wavelength characteristics are relatively flat, as compared to
the gain versus wavelength characteristics of the Al-low-density EDFA shown in FIG. 7. However, the gain versus wave-
length characteristics of the Al-high-density EDFA has a third gain peak between the 1.54-nm band and the 1 .555-^m
band.
Therefore, although the Al-high<iensity EDFA may provide wide band transmission characteristics, when a number
55 of optical amplifier are connected in series, ripple may occur in the signal transmission band.
FIG. 9 is a graph illustrating calculated transmission characteristics of a WDM signal in a case where twenty (20)
Al-hlgh-density EDFAs are connected In series. Gain dispersion shown in FIG. 9 may be reduced by a gain equalizer
according to embodiments of the present invention.
The following is a detailed description of a gain equalizing operation for the gain versus wavelength characteristics
6
BNSDOCID: <EP p794599A2J_>
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of the Al-lcw-density EDFA showmn FiG. 7 and the gain versus wavelength characteristics of the Al-high-density EDFA
shown in FIG. 8,
As shown in FIG. 7. the gain versus wavelength characteristics of the Al-low-density EDFA has two peaks. In this
case, a gain equalizer according to embodiments of the present invention may be constructed with a first Fabry-Perot
etalon filter (such as, for example, optical fitter 20-1) having transparency characteristics represented by a periodic
waveform having a period related to the wavelength difference between the two peaks, and a second Fabry-Perot
etalon filter (for example, optical filter 20-2) having transparency characteristics represented by a periodic waveform
having a period equal to one-half the period of the waveform of the first Fabry-Perot etalon filter
FIG. 10 is a graph Illustrating transparency characteristics of a gain equalizer (GEQ) and an equalized total gain in
a case where Fabry-Perot etalon filters of 32-nm and 16-nm periods are connected in series (for example, see optical
filters 20-1 and 20-2 in FIG. 3), according to an embodiment of the present invention. To easily understand the operation
of a gain equalizer, the gain versus wavelength characteristics (EDFA gain) of the Al-low-density EDFA shown in FIG.
7 are also represented in FIG. 10.
In FIG- 10, the transparency characteristics in a 1533-nm to 1560-nm band may be substantially equal to the
reverse characteristics of the gain versus wavelength characteristics of an Al-low-density EDFA. with optical filters used
as a gain equalizer. In this case, an approximate equation with respect to transparency T(X) of a Fabry-Perot etalon filter
is represented as follows:
20
25
T(X)=lOlogio
lOlog
10
{1-Ri)'
2 2f2rt(>^->^0)
(l-R^) +4xRi XCOS J p^^g +t|)ixrt
(l-R^)^
(l-Rg)^ +4 X R2 X sin^
2ti(X-Xo)
30
Xo = 1533,
Ri=:0.1 F^=32, (|)i=0.0
R2 = 0.1 F2 = 16, (t»2 = 0.4
35
in this manner, the amplifier characteristics (EDFA gain) of an Al-low-density EDFA may be equalized by the trans-
parency characteristics of the gain equalizer. From the equalizing result (total gain), by combining two different period-
ical optical filters (a first optical filter having transparency characteristics represented by a periodical waveform having
a period related to the wavelength difference between peaks of the amplifier characteristics, and a second optical filter
40 having transparency characteristics represented by a periodic waveform with a period substantially equal to one-half of
the period of the waveform of the first optical filter), the gain versus wavelength characteristics of the Al-low-density
EDFA may be flattened in an approximately 15-nm wide wavelength band.
On the other hand, as shown in FIG. 8. the gain versus wavelength characteristics of an Al-high-density EDFA has
three peaks. In this case, a gain equalizer according to the embodiments of the present invention may be formed by
45 three Fabry-Perot etalon filters (such as optical filters 20-1 , 20-2 and 20-3 illustrated in FIG. 3) where the transmission
characteristics of a first optical filter has transparency characteristics related to the wavelength difference between
peaks of the amplifier characteristics, a second optical filter has transparency characteristics represented by a periodic
waveform having a period substantially equal to one-half of the period of the waveform of the first optical filter, and a
third optical filter has transparency characteristics represented by a periodic waveform with a period substantially equal
50 to one-quarter the period of the waveform of the first optical filter.
FIG. 1 1 is a graph illustrating transparency characteristics of a gain equalizer (GEQ) and an equalizing result in a
case where Fabry-Perot etalon filters of 56-nm, 28-nm and 14-nm periods are connected in series, according to an
embodiment of the present invention. To easily understand the operation of the gain equalizer, the gain versus wave-
length characteristics (EDFA gain) of the Al-high-density EDFA shown in FIG. 8 is also represented in FIG. 1 1 .
55 In FIG. 11, the transparency characteristics in a 1533-nm to 1560-nm band may be substantially equal to the
reverse characteristics of the gain versus wavelength characteristics of an Al-high-density EDFA, with optical filters
being used as a gain equalizer. In this case, an approximate equation with respect to transparency T(>.) of a Fabry-Perot
etalon filter is represented as follows:
7
BNSOOCIO: <EP p79459eA2J_>
EP 0 794 599 A2
T{X)=10logio
(i-Ri)
10log
(1-Ri)^ +4xRi xcos^
(l-Ra)'
2n(X-Xo)
FiX2
10
(l-Rg)^ +4xR2 xsin^
2n(X-Xo) ^
10log
10
(I-R3)'
{I-R3) +4XR3XCOS I F^x2 '^'''2^"]
Xo a 1532
Rl = 0.040 F, = 56 <t>i = 0.0
R2 = 0.025 Fa = 28 <|)2 = 0.6
R3 = 0.045 F3 = 14 (1)3 = 0.0
in this manner, the amplifier characteristics (EDFA gain) of an Al-hlgh-density EDFA may be equalized by the trans-
parency characteristics of a gain equalizer. From the equalizing result (total gain), by combining three different period-
ical optical filters, the gain versus wavelength characteristics of an Al-high-density EDFA may be flattened in an
approximately 30-nm wide wavelength band. For example, a first optical filter can have transparency characteris^cs
represented by a periodic waveform having a period related to the wavelength difference between peaks of the amplifier
characteristics, a second optical filter can have transparency characteristics represented by a periodic waveforn, with a
period substantially equal to one-half of the period of the waveform of the first optical filter, and a third optical filter can
have transparency characteristics represented by a periodic waveform with a period substantially equal to one-quarter
the period of the waveform of the first optical filter.
in the various proposed method for equalizing gain as discussed in the Background of the Invention section, a sin-
gle band adjacent to one gain peak is used, and a resulting flattened band is approximately 10 nm. By contrast, a gam
equalizer according to embodiments of the present invention can provide a much wider gain band by using more than
one peak to flatten gain. ., u- u j cncA
FIG. 12 is a diagram illustrating a gain equalizer for. equalizing amplifier characteristics of an AI-high-densrty EDFA,
according to an embodiment of the present invention, and which is similar to the gain equalizer illustrated in FIG. 3^ In
FIG 12 an amplHication and equalization device 2000 includes optical filters 20-1. 20-2 and 20-3. Optical filters 20-1
20-2 and 20-3 are illustrated with solid lines to indicate a case where the optical filters are provided at the input side of
Al-high-density EDFA 10. and optical filters 20-1 . 20-2 and 20-3 are illustrated with dotted lines to indicate a c^e where
the optical filters are provided at an output side of Al-high-density EDFA 1 1 . As illustrated in FIG. 12. optical filters 20-
1.20-2 and 20-3 are connected In series. ... ^ .
A portion of the \NDM signal can be decoupled and detected by a photo detector 18. which provides an output sig-
nal to pumping LD controller 1 6. LD controller 1 6 can then determine characteristics of the WDIVI signal from the decou-
pled portion and control optical filters 20-1. 20-2. 20-3 to adjust parameters such as period, phase, and amplitude
(attenuation) of the waveform of the transparency characteristics. Theses adjustment can be performed in a Fabry-
Perot etalon filter by adjusting the incident angle of the light, and mechanically adjusting the thickness L of a spacer
FurSe/'when the spacer layer has previously been formed so that the thickness varies In a horizontal direction,
substantially the same adjustment may be performed by changing (sliding) an incident position of the light. In a case
when the gain versus wavelength characteristics of the EDFA varies due to age softening, and in a case when the EDFA
has malfunction, these parameters can then easily be adjusted. , r^ ^ 1
Therefore according to the above embodiments of the present Invention, a controller (such as pumping-LD control-
ler 16) controls optical filters of a gain equalizer to adjust the period, phase and/or attenuation degree of the waveforms
of the transparency characteristics of the optical filters.
IVIoreover by arranging a plurality of optical filters in series having transparency characteristics with different pen-
ods. phase, a^ amplitude (attenuation), as Illustrated in FIG. 12. the amplifier characteristics of an EDFA having a plu-
rality of gain peaks can be equalized In a wider wavelength band.
8
I: <£P .07945MA3
tEP 0 794 599 A2
_ a gain equalizer for equalizing amplifier characteristics of an Al-high-density EDFA,
according to an additional embodiment of the present invention. Referring now to FIG. 13. an amplification and equali-
zation device 3000 includes optical filters 20-1. 20-2 and 20-3 connected in parallel.
A WDM signal is divided by branching devices 80 and 82 into three separate signals. A first signal passes through
5 a first Al-high-density EDFA 10-1. A second signal passes through a second Al-high-density EDFA 10-2. Similarly, third
signal passes through a third Al-high<lensity EDFA 10-3. Thus, branching devices 80 and 82. taken together, can be
considered to be a branching device which branches the WDM signal into first, second and third signals which are each
passed through a different EDFA. Al-high-density EDFAs 10-1. 10-2 and 10-3 preferably have substantially the same
gain versus wavelength characteristics.
10 The first signal, amplified by Al-htgh-density EDFA 10-1 . is filtered by optical filter 20-1 having transparency char-
acteristics represented by a periodic waveform with a period related to a wavelength difference between peaks of the
amplifier characteristics, as previously described. The second signal, amplified by Al-high-density EDFA 1 0-2. is filtered
by optical filter 20-2 having transparency characteristics represented by a periodic waveform with a period equal to one-
half the period of the waveform of the transparency characteristics of optical filter 20-1. as previously described. The
15 third signal, amplified by Al-high-density EDFA 1 0-3. is filtered by optical filter 20-3 having transparency characteristics
represented by a periodic waveform with a period equal to one-quarter the period of the waveform of the transparency
characteristics of optical filter 20-1 , as previously described.
The filtered, amplified signals produced by optical filters 20-1. 20-2 and 20-3 are then combined into a resulting sig-
nal which is output as an amplified WDM signal. To properly combine the filtered amplified signals, the phases of filtered
20 amplified signals to be combined are adjusted by phase adjusters 30-1 . 30-2 and 30-3. A conventional optical coupler
can easily perform such a combining operation.
In FIG. 13. optical filters 20-1 , 20-2 and 20-3 are positioned in parallel and after the corresponding Al-high-density
EDFA 10-1. 10-2 and 10-3. respectively. However, optical filters 20-1. 20-2 and 20-3 can be positioned in parallel and
before the corresponding Al-high-density EDFA 10-1. 10-2 and 10-3. respectively
25 In FIG. 13, branching devices 80 and 82 branch the WDM signal into first, second and third signals before the WDM
signal is amplified. The first second and third signals are then individually amplified and filtered. However, the embod-
iments of the present invention are not intended to be limited to branching the WDM signal before the WDM signal is
amplified.
For example FIG. 14 is a diagram illustrating a modification of the gain equalizer illustrated in FIG. 13. according
30 to an embodiment of the present invention. As illustrated in FIG. 14. a single EDFA 10 is used to amplify a WDM signal.
Branching devices 80 and 82 are positioned after EDFA 10. Therefore, branching devices 80 and 82 branch the ampli-
fied WDM signal into first, second and third signals which are provided to optical filters 20-1 . 20-2 and 20-3, respec-
tively. The filtered signals are then combined together into an amplified, filtered WDM signal.
FIG 15 is a diagram illustrating a further modification of the gain equalizer illustrated in FIG, 13. according to an
35 embodiment of the present invention. As illustrated in FIG. 15. a single EDFA 10 is used to amplify a WDM signal.
Branching devices 80 and 82 and filters 20-1. 20-2 and 20-3 are positioned before EDFA 10. Therefore, branching
devices 80 and 82 branch the WDM signal into first, second and third signals which are provided to optical filters 20-1 .
20-2 and 20-3. respectively. The filtered signals are then combined together into a filtered WDM signal which is then
amplified by EDFA 10.
40 FIGS. 14 and 15 are simplified drawings and do not show a pumping-LD controller, associated laser diodes and
associated photo detectors. However, such components, as illustrated in FIG. 13. can be included in the configurations
illustrated In FIGS. 14 and 15. Moreover, phase adjusters 30-1, 30-2 and 30-3 are not illustrated in FIG. 15 but. if
desired, can be provided after optical filters 20-1 . 20-2 and 20-3. respectively
By connecting a plurality of optical filters in parallel having different periods, phase, and amplitude (attenuation) of
45 transparency characteristics, as illustrated in FIG. 1 3. 1 4 and 1 5. the transparency characteristics of a plurality of optical
filters can be combined. Therefore, such a configuration will allow the amplifier characteristics of an EDFA having a plu-
rality of gain peaks to be equalized in a wider wavelength band.
Moreover, by connecting a plurality of periodical optical filters in parallel and then combining the output of the opti-
cal filters, as illustrated in FIGS. 13. 14 and 15. dispersion of the gain versus wavelength characteristics of an optical
50 amplifier having a plurality of gain peaks can be equalized in a relatively wide wavelength band.
As an index indicating an excited state of an EDFA. an inversion parameter, which is a ratio of an erbium-ion density
in an excited state to a total erbium-ion density, is commonly used. Further, in an EDFA. even if in input signal is pro-
vided to the EDFA, a light is spontaneously emitted from the EDFA. This spontaneously emitted light is typically referred
to as "amplified spontaneous emission" (ASE).
55 In the following description, a case when the inversion parameter is less than 0.6 is referred to as a "saturation
state", and a case when the inversion parameter is more than 0.6 is referred to as an "unsaturated state". Now. consid-
ering the amplifier characteristics of an EDFA, in an Al-low-density EDFA. ASE-3dB wavelength bands indicating a
quantity index of the signal transmission band are substantially the same. i.e.. approximately 10-nm in both the satura-
tion state and unsaturated state. On the other hand, in an Al-high-density EDFA. the ASE-3dB wavelength band in the
9
EP 0 794 599 A2
10
15
20
25
30
35
40
45
50
55
unsaturated State is Wider than that in the saturation State.
Therefore when a tens-of-nm wavelength wide arrplifying band having little gain dispersion -s require! use of an
W-hiS^SS EDFA operating in the unsaturated state is more effective. However, ^^^^^
l?nte unsaturated state are connected in a multiple-stage form, a 1 .53 .m band ASE .ncreases, and a large ga.n
roclTurt SoS. When using an Al-high<lensity EDFA operating in the unsaturated state, the 1 .53 ^m band ASE
"'In Se unSSra^d state, an EDFA generally has characteristics which amplify optical power j^^ticularly in the 1 .53
^m bind Sore, if the optical power in this band is initially small, degradation of the overall optical SNR may be
'^"fS' 16 is a diagram illustrating a gain equalizer for equalizing amplifier characteristics of an Al-high-density EDFA,
accSto a?ur S e^Sme^^^^^ L pr^ent invention. Referring now to FIG. 1 6, an amplification and equalization
^4000 fn^de^ gain equalizer 20 and a 1.53 .m band attenuation (notch) filter 40. Attenuation ^-ter 40 operates
attenuator andls provided between gain equalizer 20 and an output terminaL Attenuation filter 40 allows
dearadation of the optical SNR in the next stage optical amplHier to be signif icantiy reduced.
Therefore atlast one of a plurality of gain peaks.of an optical amplHier can be attenuated by using an attenuation
filter S^ch use d a^a^^^^^ filter is effective for use in an optical communication system in which spontaneously
gfnerarno'e leleKASE) light in a specified wavelength band increases. Therefore, the use of an attenuation filter will
reduce degradation of optical signal to noise ratio (SNR). ^^^r^rHinn m an
FIG. 17 is a diagram illustrating an optical transmission system which ^^^^^^^^^
embodiment of the present invention. Referring now to FIG. 1 7. an optical sending station (OS) 52setKls an optical sig
n^^ras a WD.5 signal, through transmission line 5 to an optical receiving station Of^Jf^SStersfo 20 2
to 50-n (for a total of "n" optical amplifiers) are connected in series along transmission line 5. Opticamters 20-1 , 20 2,
20 3 £a tSnsm ssion characteristics with different periods (as previously described), are arranged at proper inter-
Sls from e'rorer Tong transmission line 5. Such an optical transmission system can simultaneously equalize a
multSe form, t^^^^^^^ gain equalize^compri^ng the plurality of optical filters) to equalize gam dispersion
^^'^ni^XlSSng^^
ina to an additional embodiment of the present invention. Referring now to FIG. 18, n optical amplifiers 50-1 to 50-n are
Sir^e^te^t Ss al^^^ transmission Hne 5. Gain equalizers 20#1 to 20#3 are arranged at appropriate '"tervals aicmg
rar^mis^on Hne 5 Eac^ gain equalizer 20#1 . 20#2 and 20#3 includes optical filters for ^'f ^ain of or^ical
am^ifiers 50-1 to 50-n. Fo? example, each gain equalizer 20#1, 20#2 and 20#3 includes optical ''^^^^ 20;^'^20-2_
Se FIG 3) having transparency characteristics with different periods. In FIG. 1 8, gam equalizers 20#1 , 20#2 and 20#3
M^eot? ior'i'Sple each gain equalizer 20#1. 20#2 and 20#3 can compensates for. as a whole, the character-
istics Of a! optiS am^Sers located atVhe preceding stages of. and the following stages of. the respective gam equal-
'''\he optical transmission system illustrated in FIG. 18 is effective when the optical SNR of each channel of
sianal is to be equalized as the WDM signal travels along a path of the optical transmission system. For example the
ODtSnrlnsmiSon system illustrated in FIG. 18 is effective when portions of the WDM signal light are branched from
me SnSoT^th S When various signals are combined together into a WDM signal light which travels along the
''"TclTis'a diagram illustrating an optical transmission system which includes a plurality of amplif ication and eq^^l-
izatioTdelles according to an eiodiment of the present invention. Referring now to FK3. 19, " oPtica. ampl^^^^^^^ 50^
1 to 50-n are connected in series along transmission line 5. Amplification and equalization devices 60#1 to 60#3 are
arranaed at appropriate intervals along transmission line 5. Each amplification and equalization device 60#1 to 60#3
rnrS?rf«TfortSmoritical filters 20-1 20-2. 20-3 connected in parallel as shown in FIG. 13. Thus, each amplrfica ion
X J::rntr ^^^^^^^ -.s configured as, for example, amplification and equalizat.n ^^^^2:^:.
tratedin FIG. 13. Such an optical transmission system can simultaneously equalize a WDM signal passed through a
'"torelTS ramlS'each amplication and equalization device 60#1 to 60#3 can compensates ton as a who,,
the TlZi'ZL^^^o^^c^^ amplHiers located at the preceding stages of. and the followmg stages of, the respective
'"^SSe'fcSSt^^^^^^^^ embodiments of ti,e present invention, a communication system connecj an
optical send ng station to an optical receiving station so that an optical signal sent by the optical sending station is
?ei1Je^ by the optical receiving station. The communication system includes a plurality of optica amplifiers whK:h each
LrS if^ he S signal sent by the optical sending station before being received by the optical receiving station. The
SnSS «mpl«iers have a combined gan versus wavelength characteristic with first and second gam peaks
10
BNSOOCIO: <EP ^079459«A2_L>
EP 0 794 599 A2
in a wavelength band and a wSHirigth difference between the first and second gain peaks. The communication sys-
tem also includes a plurality of filters which filter the optical signal sent by the optical sending station before being
received by the optical receiving station. Each filter has a transparency characteristic which is a periodic waveform hav-
ing a period related to the wavelength difference between the first and second gain peaks. The filters can be included
in various equalizers, where each equalizer has filters connected either in series or in parallel.
The wavelength, phase, and amplitude (attenuation) of the transparency characteristics of each gain equalizer
shown in FIGS. 17, 18, and 19 are adjustable to allow an overall optical SNR in the transmission system to be equal-
ized. Preferably, such adjustment can be made in accordance with a command signal produced from a remote location
(not illustrated). Such adjustment can be performed if a Fabry-Perot etalon filter, shown in FIG. 4, is used as an optical
filter. More specifically, a Fabry-Perot etalon filter will allow the period, phase, and amplitude (attenuation) of the trans-
parency characteristics of each gain equalizer to be controlled by adjusting the incident angle of light and mechanically
adjusting the thickness L of a spacer layer (see spacer layer 100 in FIG. 4). In addition, when the spacer layer has been
previously formed so that the thickness varies in a horizontal direction, substantially the same adjustment can be per-
formed by changing (sliding) an incident position of the light.
Therefore, based on the command produced from a remote location, the period, phase, and attenuation degree of
the transparency characteristics of an optical filter can be adjusted. As a result, when the transparency characteristics
of the optical amplifier varies due to age softening, or when the optica! amplifier malfunctions, the transparency charac-
teristics of the gain equalizer can be properly adjusted.
According to the above embodiments of the present invention. Al-high-density EDFAs are used as optical amplifi-
ers. However, the present invention is not intended to be limited to use with any specific density EDFA. Many different
densities can be used in various embodiments of the present invention. Moreover, the various embodiments of the
present invention are not intended to be limited to use with an EDFA. and many other types of optical amplifiers can be
used. For example, optical amplifiers can be formed by doping a fiber with rare earth elements other than Erbium.
According to embodiments of the present invention as described above, amplifier characteristics having periodical
peaks may be equalized in a wide wavelength band by properly combining a plurality of optical filters having different
transparency characteristics (that is, different periods, phase, or amplitude (attenuation)).
According to the above embodiments of the present invention, a gain equalizer includes optical filters having trans-
parency characteristics represented by periodic waveforms with different periods. Various examples of the periods are
also provided herein. For example, as illustrated in FIGS. 5(B). 5(C) and 5(D). a second optical filter has transparency
characteristics represented by a waveform with a period which is 1/2 the period of a waveform representing transpar-
ency characteristic of a first optical filter. Similarly, a third optical filter has a transparency characteristics represented
by a waveform with a period which is 1/4 the period of a waveform representing transparency characteristic of the first
optical filter However, the present invention is not intended to be limited to these specific periods. Instead, other relative
periods can be used.
Although a few preferred embodiments of the present invention have been shown and described, it would be appre-
ciated by those skilled in the art that changes may be made in these embodiments without departing from the principles
and spirit of the invention, the scope of which is defined in the claims and their equivalents.
1. An apparatus for equalizing gain versus wavelength characteristics of an optical amplifier, the gain versus wave-
length characteristics having first and second gain peaks in a wavelength band with a wavelength difference
between the first and second gain peaks, the apparatus comprising:
first and second optical filters connected to the optical amplifier and having first and second transparency char-
acteristics, respectively, the first and second transparency characteristics being periodic waveforms having
periods related to the wavelength difference between the first and second gain peaks, the period of the wave-
form of the first transparency characteristic being different from the period of the waveform of the second trans-
parency characteristic.
2. An apparatus as in claim 1. wherein the gain versus wavelength characteristics of the optical amplifier include a
third gain peak between the first and second gain peaks, and the apparatus further comprises:
a third optical filter connected to the optical amplifier and having a third transparency characteristic which is a
periodic waveform having a period related to the wavelength difference between the first and second gain
peaks, the period of the waveform of the third transparency characteristic being different from the periods of
the waveforms of the first and second transparency characteristics.
3. An apparatus as in claim 1, wherein the first transparency characteristic is a periodic waveform with 1/4 period
Claims
11
<EP q794S99A2_l_>
EP 0 794 599 A2
extending between the first anc^H^d gain peaks.
4. An apparatus as in claim 3, wherein the second transparency characteristic is a periodic waveform with one period
extending between the first and second gain peaks.
' 5
5. An apparatus as in claim 2, wherein:
the first transparency characteristic is a periodic waveform with 1/4 period extending between the first and sec-
ond gain peaks,
10 the second transparency characteristic is a periodic waveform with one period extending between the first and
second gain peaks, and
the third transparency characteristic is a periodic waveform with two periods extending between the first and
second gain peaks.
15 6. An apparatus as in claim 1 , wherein the second transparency characteristic is a periodic waveform having a period
equal to 1/(2") of the period of the waveform of the first transparency characteristic.
7. An apparatus as in claim 1 , wherein the second transparency characteristic is a periodic waveform having a period
equal to 1/2 of the period of the waveform of the first transparency characteristic.
20
8. An apparatus as in claim 7, wherein the gain versus wavelength characteristics of the optical amplifier include a
third gain peak between the first and second gain peaks, and the apparatus further comprises:
a third optical filter connected to the optical amplifier and having a third transparency characteristic which is a
25 periodic waveform having a period related to the wavelength difference between the first and second gain
peaks and which is equal to 1/4 of the period of the waveform of the first transparency characteristic.
9. An apparatus as in claim 1 . wherein at least one of the first and second optical filters is a Fabry-Perot etalon filter.
30 10. An apparatus as in claim 1 , wherein:
the optical amplifier amplifies an input signal in accordance with the gain versus wavelength characteristics to
produce an output signal, and
the first and second optical filters are arranged in series to filter one of the group consisting of the input signal
35 and the output signal.
11 . An apparatus as in claim 8, wherein:
the optical amplifier amplifies an input signal in accordance with the gain versus wavelength characteristics to
40 produce an output signal, and
the first, second and third optical filters are arranged in series to filter one of the group consisting of the input
signal and the output signal.
12. An apparatus as in claim 6, further comprising:
45
a branching device which branches an optical signal into a first signal and a second signal, the first and second
optical filters being in parallel and the first signal being filtered by the first optical filter and the second signal
being filtered by the second optical filter ; and
a combining device which combines the filtered first and second signals into a combined signal which is ampli-
50 fied by the optical amplifier.
13. An apparatus as in claim 12, the gain versus wavelength characteristics of the optical amplifier including a third gain
peak between the first and second gain peaks, and the apparatus further comprises:
55 a third optical filter having a third transparency characteristic which is a periodic waveform having a period
related to the wavelength difference between the first and second gain peaks and which is equal to 1/4 of the
period of the waveform of the first transparency characteristic, wherein
the branching device branches the optical signal into the first signal, the second signal and a third signal,
12
BNSOOCID: <EP 0794599A2_L>
15
EP 0 794 599 A2
the first, second aHBd optical filters are in parallel and the third signafii filtered by the third optical filter,
and
the combining device combines the filtered first, second and third signals into the combined signal.
5 14. An apparatus as in claim 6. the optical amplifier amplifying an input signal to produce and output signal and the
apparatus further comprises:
a branching device which branches the output signal into a first signal and a second signal, the first and second
optical filters being in parallel and the first signal being filtered by the first optical filter and the second signal
10 being filtered by the second optical filter, and
a combining device which combines the filtered first and second signals into a combined signal.
1 5. An apparatus as in claim 1 4. the gain versus wavelength characteristics of the optical amplifier including a third gain
peak between the first and second gain peaks, and the apparatus further comprises:
a third optical filter having a third transparency characteristic which is a periodic waveform having a period
related to the wavelength difference between the first and second gain peaks and which is equal to 1/4 of the
period of the waveform of the first transparency characteristic, wherein
20 the branching device branches the output signal into the first signal, the second signal and a third signal.
the first, second and third optical filters are in parallel and the third signal is filtered by the third optical filter.
mTcombining device combines the filtered first, second and third signals into the combined signal.
25 16. An apparatus as in claim 14. further comprising:
a first phase adjuster which adjusts the phase of filtered first signal before the filtered first and second signals
are combined; and . ^ .
a second phase adjuster which adjusts the phase of filtered second signal before the filtered first and second
30 signals are combined.
17. An apparatus as in claim 1 , further comprising:
an optical attenuator which attenuates at least one of the first and second gain peaks.
18. An apparatus as in claim 17. wherein the optical attenuator is a notch filter.
19. An apparatus as in claim 1. further comprising:
40 a controller which
controls the first optical filter to adjust at least one of the group consisting of period, phase and attenuation
degree, of the waveform of the first transparency characteristic, and
controls the second optical filter to adjust at least one of the group consisting of period, phase and atten-
ds uation degree, of the waveform of the second transparency characteristic.
20. An apparatus as in claim 1 . wherein:
the optical amplifier amplifies an input signal to produce and output signal,
50 one of the first and second optical filters receives and filters the input signal, and
the other of the first and second optical filters receives and filters the output signal.
21. An apparatus as in claim 1, wherein:
55 the optical amplifier amplifies an input signal in accordance with the gain versus wavelength characteristics to
produce an output signal, and
the first and second optical filters both filter a same one of the group consisting of the input signal and the out-
put signal.
35
13
BNSOOCID; <EP ^0794599A2_L>
10
EP 0 794 599 A2
22. An optical communication sysf^^omprlslng:
an optical amplifier with gain versus wavelength characteristics having first and second gain peaks in a wave-
fenl TandTnd a wavelength difference between the first and second gain peaks, the optical amplrfier amph-
fylTanlnpursig^TrnaJdancewiththeg^
f iral and second optical filters having first and second transparency characteristicsjespectively, the Jirst and
sSTtrlS^r^cy characteristics being periodic waveforms having periods related to the wavelength drf^er-
enS blS^e f ir'st and second gan peaks, the period of the waveform of the first transparency cha^^^^^^^^^
istic being different from the period of the waveform of the second transparency <=5aracter|st.c, the f^^^^^ and
second optical filters each filtering one of the group consisting of the input signal and the output signal.
23 An optical communication system as in claim 22. wherein the second transparency characteristic is a periodic
^vSrm teving a period equal to 1/(2") of the period of the waveform of the first transparency charactenstic.
24. An optical communication system as in claim 23. wherein the first and second optical filters are m senes.
25. An optical communication system as in claim 23. wherein
20 one of the first and second optical filters receives and filters the input signal, and
the other of the first and second optical filters receives and filters the output signal.
26. An optical communication system as in claim 22, wherein the gain versus wavelength f^racteristics of the optical
ampEier include a third gain peak between the first and second gain peaks, and the optical communication system
25 further comprises:
a third optical filter filtering one of the group consisting of the input signal and the output signaUhe ^^'^^^^
filter having a third transparency characteristic which is a periodic waveform having a
wavelength difference between the first and second gain peaks, the period of the waveform of the third trans-
pa wchar^^^^^^^^^ being different from the periods of the waveforms of the first and second transparency
characteristics.
27. An optical communication system as in claim 26. wherein
the second transparency characteristic is a periodic waveform having a period equal to 1/(2") of the period of
the waveform of the first transparency characteristic, and
the third transparency characteristic is a periodic waveform having a period equal to 1/4 of the period of the
waveform of the first transparency characteristic.
40 28. An optical communication system as in claim 22. further comprising:
a branching device which branches the input signal into a first signal and a second signal, the first and seco^^^^^
optical filters being in parallel and the first signal being filtered by the first optical filter and the second signal
being filtered by the second optical filter, and u «j • ^.i .«h>h ie
a combining device which combines the filtered first and second signals into a combined signal which is pro-
vided to the optical amplifier for amplification by the optical amplifier.
30
35
29. An optical communication system as in claim 22. further comprising
50
abranching device which branches the output signal intoafirst signal andasecondsignal.thefirst an
optical filters being in parallel and the first signal being filtered by the first optical filter and the second signal
being filtered by the second optical filter, and
a combining device which combines the filtered first and second signals into a combined signal.
55 30. An optical communication system as in claim 22, further comprising:
a controller which
controls the first optical filter to adjust at least one of the group consisting of period, phase and attenuation
14
BNSDOCIO: <eP p794599A2_l_>
10
20
• EP 0 794 599 A2
of the first transparency characteristic, and
controls the second optical filter to adjust at least one of the group consisting of period, phase and atten-
uation degree, of the waveform of the second transparency characteristic.
31, An apparatus for equalizing gain versus wavelength characteristics of an optical amplifier, the gain versus wave-
length characteristics having first, second and third gain peaks in a wavelength band with the second gain peak
being between the first and third gain peaks and a wavelength difference between the first and third gain peaks, the
optical amplifier amplifying an input signal in accordance with the gain versus wavelength characteristics to pro-
duce an output signal, the apparatus comprising:
first, second and third optical filters having first, second and third transparency characteristics, respectively,
wherein
the first, second and third transparency characteristics are periodic waveforms having different periods
15 related to the wavelength difference between the first and second gain peaks,
the second transparency characteristic is a periodic waveform having a period equal to 1/(2") of the period
of the waveform of the first transparency characteristic, and
the third transparency characteristic is a periodic waveform having a period equal to 1/4 of the period of
the waveform of the first transparency characteristic.
32. An optical communication system comprising:
a branching device which branches an optical signal into first and second signals:
a first optical amplifier which amplifies the first signal;
25 a first optical filter which filters one of the group consisting of the first signal before being amplified by the first
optical anplifier and the first signal after being amplified by the first optical amplifier;
a second optical amplifier which amplifies the second signal;
a second optical filter which filters one of the group consisting of the second signal before being amplified by
the second optical amplifier and the second signal after being amplified by the second optical amplifier; and
30 a combining device which combines the amplified, filtered first and second signals into a combined signal,
wherein
the first and second optical amplifiers have substantially the same gain versus wavelength characteristics,
the gain versus wavelength characteristics having first and second gain peaks in a wavelength band with
35 a wavelength difference between the first and second gain peaks, and
the first and second optical filters have first and second transparency characteristics, respectively, the first and
second transparency characteristics being periodic waveforms having periods related to the wavelength differ-
ence between the first and second gain Peaks, the period of the waveform of the first transparency character-
40 istic being different from the period of the waveform of the second transparency characteristic.
33. An optical communication system as in claim 32. wherein the second transparency characteristic is a periodic
waveform having a period equal to 1/2 of the period of the waveform of the first transparency characteristic.
45 34. An optical communication system as in claim 32, wherein
the branching device branches the optical signal into the first signal, the second signal and a third signal,
the optical communication system further comprises
a third optical amplifier which amplifies the third signal and has substantially the same gain versus wave-
length characteristics as the first and second optical amplifiers, and
a third optical filter having third transparency characteristics and which filters one of the group consisting
of the third signal before being amplified by the third optical amplifier and the third signal after being ampli-
fied by the third optical amplifier.
the combining device combines the amplified, filtered first, second and third signals into the combined signal,
and
the third transparency characteristic is a periodic waveform having a period related to the wavelength differ-
ence between the first and second gain peaks and which is different from the period of the waveforms of the
15
BNSOOCID: <EP P794599A2_L>
EP 0 794 599 A2
first and second transpan
laracteristics.
35. An optical communication system as in claim 34, wherein the second transparency characteristic is a periodic
waveform having a period equal to 1/2 of the period of the waveform of the first transparency characteristic, and the
third transparency characteristic is a periodic waveform having a period equal to 1/4 of the period of the waveform
of the first transparency characteristic.
36. A method for equalizing gain versus wavelength characteristics of an optical amplifier which amplifies an input sig-
nal in accordance with the gain versus wavelength characteristics to produce an output signal, the gain versus
wavelength characteristics having first and second gain peaks in a wavelength band with a wavelength difference
between the first and second gain peaks, the method comprising the steps of:
filtering one of the group consisting of the input signal and the output signal with a first transparency charac-
teristic; and
filtering one of the group consisting of the input signal and the output signal with a second transparency char-
acteristic, wherein the first and second transparency characteristics are periodic waveforms having periods
related to the wavelength difference between the first and second gain peaks, the period of the waveform of
the first transparency characteristic being different from the period of the waveform of the second transparency
characteristic.
37. A method as in claim 36, wherein the gain versus wavelength characteristics of the optical amplifier include a third
gain peak between the first and second gain peaks, and the method further comprises the step of:
filtering one of the group consisting of the input signal and the output signal with a third transparency charac-
teristic which is a periodic waveform having a period related to the wavelength difference between the first and
second gain peaks, the period of the waveform of the third transparency characteristic being different from the
periods of the waveforms of the first and second transparency characteristics.
38. A method as in claim 37, wherein:
the first transparency characteristic is a periodic waveform with 1/4 period extending between the first and sec-
ond gain peaks,
the second transparency characteristic is a periodic waveform with one period extending between the first and
second gain peaks, and
the third transparency characteristic is a periodic waveform with two periods extending between the first and
second gain peaks.
39. A method as in claim 36, wherein the second transparency characteristic is a periodic waveform having a period
equal to 1/(2") of the period of the waveform of the first transparency characteristic.
40. A method for equalizing gain versus wavelength characteristics of an optical amplifier, the gain versus wavelength
characteristics having first and second gain peaks in a wavelength band with a wavelength difference between the
first and second gain peaks, the method comprising the steps of:
branching a light signal into first and second signals;
filtering the first signal with a first transparency characteristic;
filtering the second signal with a second transparency characteristic; and
combining the first and second signals into a combined signal which is amplified by the optical amplifier,
wherein the first and second transparency characteristics are periodic waveforms having periods related to the
wavelength difference between the first and second gain peaks, the period of the waveform of the first trans-
parency characteristic being different from the period of the waveform of the second transparency characteris-
tic.
41 . A method for equalizing gain versus wavelength characteristics of an optical amplifier which amplifies an input sig-
nal in accordance with the gain versus wavelength characteristics to produce an output signal, the gain versus
wavelength characteristics having first and second gain peaks in a wavelength band with a wavelength difference
between the first and second gain peaks, the method comprising the steps of:
branching the output signal into first and second signals;
16
: <EP ^0794599A2_L>
• EP 0 794 599 A2
a first transparency characteristic;
filtering the second signal with a second transparency characteristic: and ,u,,
conSning the first J second signals into a corr^ined signal, wherein the first and second ^^-^l^^^f^-
acteristics are periodic waveforrris having periods related to the wavelength difference between the f rst and
sS^nd gain peaks, the period of the waveform of the first transparency characteristic being different from the
period of the waveform of the second transparency characteristic.
42 A communication system which connects an optical sending station to an optical receiving station ^n °Pt;^«l
signaTsent by the optical sending station is received by the optical receiving station, the communication system
comprising:
a plurality of optical amplifiers which each amplify the optical signal sent by the optical sending station before
being rS^U by the optical receiving station, the plurality of optical amplifiers having a comb'n«^ga.n versus
wlvelength characteristic with first and second gain peaks in a wavelength band and a wavelength difference
between the first and second gain peaks; and . u
a plurality of filters which filter the optical signal sent by the optical sending station before being ^ec^ved by
optical receiving station, each filter having a transparency characteristic which .s a periodic waveform having a
period related to the wavelength difference between the first and second gam peaks.
43 A communication system which connects an optical sending station to an optical receiving station so that an optical
sig™ent?y the optical sending station is received by the optical receiving station, the communication system
comprising:
a plurality of optical amplifiers which each amplify the optical signal sent by the optical sending station before
SSTby the optical receiving station, the plurality of optical amplrtiers having a combined 93'" ^'ersus
SergrcCSc with first second gain peaks in a wavelength band and a wavelength difference
LfsSlg sta S, Store being received by the optical receiving station, each filter having a transparency
cLiterSc Ich is a periodic waveform having a period related to the wavelength difference between the
first and second gain peaks.
44. A communication system as in claim 43, wherein the plurality of filters of each optical equalizer are connected in
series.
45. A communication system as in claim 43, wherein the plurality of filters of each optical equalizer are connected in
parallel.
<EP 0704589A2_L>
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-20|-
SIGNAL
POWER -40
(dBm)
-60
-80
1500
FIG. 2(A)
SIGNAL POWER SPECTRUM
1520 1540 1560 1580
WAVELENGTH (nm)
1600
0
-20
SIGNAL
POWER -40
(dBm)
-60
-80
FIG. 2(B)
SIGNAL POWER SPECTRUM
1500 1520 1540 1560 1580
WAVELENGTH (nm)
1600
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FIG. 5(A)
AMPLITUDE
PEAK1 PEAK 3 PEAK 2
t i \
FIG. 5(B)
WAVELENGTH
PEAK1 PEAK 3 PEAK 2
i i i
AMPLITUDE t.
AHENUATIONl-
I I
FIG. 5(C)
WAVELENGTH
PEAK1 PEAK 3 PEAK 2
\ * \
AMPLITUDE t-
AHENUATION^
FIG. 5(D)
AMPLITUDE i-
AHENUATIONj-
WAVELENGTH
PEAK1 PEAK 3 PEAK 2
t t t
FIG. 5(E)
AMPLITUDE t
ATTENUATION
WAVELENGTH
PEAK1 PEAK 3 PEAK 2
\ \
FIG. 5(F)
AMPLITUDE
WAVELENGTH
PEAK1 PEAK 3 PEAK 2
\ \ *
WAVELENGTH
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FIG. 6(A)
FIG. 6(B)
FIG. 6(C)
PEAK 1 PEAK 1 PEAK 1
♦ * ♦
1.^
FIG. 7
0
-20 h
SIGNAL
GAIN (dB)-40
-60
-801-
1500
1520 1540 1560 1580
WAVELENGTH (nm)
1600
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FIG. 8
-20
SIGNAL
GAIN (dB) -40
1500
1520 1540 1560 1580
WAVELENGTH (nm)
1600
FIG. 9
SIGNAL POWER SPECTRUM
Oh
-20
SIGNAL
POWER -40
(dBm)
-60
-80-
1500
1520 1540 1560 1580
WAVELENGTH (nm)
1600
24
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FIG. 10
EDFA
GAIN (dB)
40
30
20
10
EDFA (2 PEAKS)
—-EDFA GAIN (2 PEAKS)
—GEQ (2 FILTERS)
^TOTAL GAIN
\! - ' '
V^i^^V 1-
r i\\ !
1 1 \\ '
III' \
1 1 ' !
Ill
111'
■ — — t > — '
1500 1520 1540 1560 1580 1600
WAVELENGTH (nm)
40
FIG. 11
EDFA (3 PEAKS)
30
EDFA
GAIN (dB)
20
10
■10
1500
—-EDFA GAIN (3 PEAKS)
—GEO (3 FILTERS)
TOTAL GAIN
■ ■ 1
t 1
1 1
^ 1 1 V> X
1 1 1 ^
1 j J
) 1 I
I ' 1 I
• ■ •
1
1
t
1
1520 1540 1560 1580
WAVELENGTH (nm)
1600
25
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32
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